mobile cache support

[coreykn]

Instead of bypassing the cache for a few defined mobile themes offer the ability to create a separate cached page for mobile devices. This can be added as a cache behavior variant setting. wp_is_mobile() can be used when caching the page and the User-Agent checks in this function can be used when trying to deliver the cache.

WordPress topics:

• https://wordpress.org/support/topic/will-wp_is_mobile-be-supported/
• https://wordpress.org/support/topic/feature-request-770/
• https://wordpress.org/support/topic/feature-request-741/